1. Field of the Invention
The present invention relates to a fluorescence endoscope which is used for directly visually observing a living tissue, using fluorescence which is radiated from the living tissue when visual light is impinged upon the living tissue.
2. Description of the Related Art
In general, in a fluorescence endoscope, an exciting light filter is provided in an illumination light path defined between a light source (lamp) and an object to be examined. Exciting light, adapted to radiate fluorescence from the living tissue, can be transmitted through the endoscope. A fluorescence observing filter (fluorescence filter) is provided in the illumination light path. The fluorescence observing filter permits fluorescence radiated from the living tissue, in response to the exciting light and having a wavelength longer than that of the exciting light, to pass therethrough and does not permit the exciting light to pass through the fluorescence filter.
The wavelength band of the exciting light which excites the living tissue to radiate fluorescence therefrom is approximately 420 nm to 480 nm and the optimal wave length band for the radiation of fluorescence is approximately 450 nm to 475 nm.
On the other hand, the wavelength band of the fluorescence radiated from the living tissue is approximately 480 nm to 600 nm and the peak value of the intensity thereof is obtained at 480 nm to 520 nm, adjacent to the wavelength band of the exciting light.
Since the intensity of the fluorescence produced by the exciting light is extremely small in comparison with the exciting light, even a small amount of exciting light which reaches the observing portion makes it difficult to observe the fluorescence. To this end, the exciting light filter and the fluorescence filter must be produced so that the wavelength band which can pass through the exciting light filter and the wavelength band which can pass through the fluorescence filter do not overlap.
However, since the wavelength bands of the exciting light and the fluorescence are adjacent as mentioned above and they exhibit the peak values of the intensity within limited ranges, if the exciting light transmission range of the exciting light filter is located away from the fluorescence transmission range of the fluorescence filter, the amount of light transmitted through the filter is reduced, so that the intensity of the fluorescence to be observed is weakened, thus resulting in a diagnosis error.
In theory, it is ideal that the exciting light filter and the fluorescence filter are prepared so that the longest wavelength of the light transmitted through the exciting light filter is identical or almost identical to the shortest wavelength of the light transmitted through the fluorescence filter.
The longest wavelength or the shortest wavelength transmitted through the filter refers to the wavelength at which the transmittance of the filter is zero, and hence it is extremely difficult to optically measure the longest wavelength or shortest wavelength. This is the reason that it is very difficult to produce the filter with reference to the longest wavelength that can pass through the exciting light filter or the shortest wavelength that can pass through the fluorescence filter.